Disclosed is a process for forming thin film transistors (TFTs) in semiconductor devices. More specifically, illustrated herein is a method of reverse printing, which involves both deposition and patterning of organic semiconductor layers in thin film transistor devices.
Electronic components, including thin film transistors (TFTs) are commonly formed on silicon-based materials. However, semiconductor devices with organic active layers and printed electronic components are emerging as an inexpensive alternative to silicon-based devices. The performance of organic-based devices may not match the performance of their silicon-based counterparts in terms of device density and reliability under extreme conditions, for example, high or low temperatures. However, in certain applications, these shortcomings can be traded off for economic benefits because organic materials provide the advantage of producing these devices without the expensive steps associated with silicon processing. Other advantages of organic-based devices include the greater mechanical flexibility and easier electronic tunability.
Forming of patterned organic semiconductor layers, which includes deposition and patterning of organic semiconductor layers, is a significant part of TFT fabrication. The deposition method has significant effects on the physical properties of the resultant semiconductor layer such as the thickness, surface roughness, film morphology. These in turn have significant effect on the performance of TFTs. Patterning of semiconductor layers is also significant, as proper patterning and isolating the semiconductor layers into discrete areas may significantly reduce device leakage.
Formation of patterned organic semiconductor layers may be achieved by deposition of semiconductor layers with vacuum deposition, spin coating, dip coating, bar coating methods, followed by patterning with conventional photolithographic method. Other methods such as vacuum deposition through a shadow and lift-off patterning, controlling surface energy and spin or dip coating to generate patterned organic semiconductor layers have also been reported. However, these methods are generally multiple-step processes or complicated. They may not therefore be suitable for manufacturing low-cost, large-area devices. An example of the above-cited deposition and patterning methods to form a patterned organic semiconductor layer is disclosed by H. E. Katz in U.S. Pat. No. 6,403,397. The '397 process involves treating a surface to selectively provide regions of greater affinity and lesser affinity for an organic semiconductor solution. When the organic semiconductor, or solution comprising the semiconductor, is deposited on the treated surface, either the organic semiconductor or the organic semiconductor solution de-wets from the lesser affinity regions or the resultant film adheres only weakly to the lesser affinity regions such that selective removal is readily achieved.
Other references in the field include: C. D. Dimitrakopoulos, et al., U.S. Pat. No. 5,946,551; K. Amundson, et al., U.S. Pat. No. 6,312,971; Suzuki, et al., U.S. Pat. No. 5,079,595.
A method including both deposition and patterning is disclosed by K. E. Paul in Appl. Phys. Lett. 2003, Vol 83, p 2070–2072. Patterned organic semiconductor layers are formed by directly printing solution of a semiconductor into the channels of TFTs via an inkjet printer. However, the method requires a specialized semiconductor ink that possesses proper viscosity and stability suitable for printing.
Thus, there is a need for a new process to deposit and pattern organic semiconductor layers in organic electronic devices.